The invention concerns the reconstruction of a three-dimensional image of an object covered by a contrast medium.
Its application is of particular interest in the medical field, in which reconstruction of the internal structures of patients under examination is undertaken and, in particular, the reconstruction of angiographic images, that is, obtaining images of opacified vasculatures by injection of a contrast medium.
The invention can, nevertheless, have applications in other fields, notably, in nondestructive industrial control, in which examinations of the same type as medical examinations are performed.
In the medical field, two-dimensional projected images of the object, for example, a patient's head, are generally obtained by rotation of an X-ray camera turning around the object. There are essentially two types of reconstruction algorithms in X-ray imaging. A first type provides for a calculation by back projection and filtering or even a Fourier transform reconstruction in several dimensions. A second type concerns the iterative methods of reconstruction, also called algebraic. The principle of such an algebraic algorithm is disclosed, for example, in French Patent Applications Nos. 8903606, 8916906 or 9807371, which describes an application of an iterative algorithm of algebraic reconstruction of images on a multi-resolution volume.
In short, after a calibration of the apparatus used to determine, notably, the parameters of projection in the projection planes of the acquired images, of an observed volume broken down into elementary volume elements or voxels (those calibration parameters forming projection matrices), the algebraic image reconstruction algorithm is used to reconstruct the three-dimensional volume from those two-dimensional projected images. The basic principle of that algorithm is to initialize the voxels of the volume to a predetermined initial value, for example, a zero value, and to iterate a number of times the following operations: projection of voxels in the plane of each acquired image so as to obtain a virtual image, determination of the difference between the projected volume (virtual image) and the corresponding acquired image, and then back projection of that difference in volume. After a number of iterations, an estimated value representative of the density of contrast medium injected in the vessels X-rayed is obtained for each voxel, which makes it possible to visualize in three dimensions the cartography of those X-rayed vessels.
Those three-dimensional images are of valuable assistance to the neurologist and the surgeon, whether for a diagnosis, planning of therapeutic procedures or evaluation of the shape and size of objects.
On the other hand, such reconstructed volume images have one major dis-advantage. Actually, they do not make it possible to visualize propagation of the contrast medium injected in the arteries, since the acquired two-dimensional images, from which the three-dimensional images has been reconstructed, are acquired for a quasi-stationary condition of the contrast medium. In fact, for the reconstruction of a three-dimensional image, which will be described here under the term “static” by reason of the quasi-stationary character of the contrast medium, it is sought rather to obtain a set of images corresponding to a same degree of propagation of the contrast medium.
Consequently, with such a reconstruction, the patient's vascular system cannot be analyzed with just three-dimensional information and blood flow information (propagation of contrast medium).